Shrimp dumplings (saeu mandu)

Name: Shrimp dumplings (saeu mandu)
Prep time: 30 min
Cook time: 10 min
Total time: 40 min
Cuisine: Korean
Ingredients: 2 cups all purpose flour (about 9 ounces/260 grams, more for dusting); 1/2 teaspoon salt; ⅔ cup water; 2 to 3 ounces (60 to 90 grams) spinach; 12 ounces (340 grams) raw shrimp (Peeled and deveined); 10 ounces (280 grams) green cabbage; 4 ounces (110 grams) fresh mushroom caps, stems removed ((shiitake, button, crimini, etc.)); 1/2 small onion; 1 teaspoon minced garlic; 1 teaspoon finely grated ginger (or juiced); 1 teaspoon soy sauce; 2 teaspoons sesame oil; salt to taste (about 1/4 teaspoon); pinch pepper; 1 tablespoon soy sauce; 1 teaspoon vinegar; 1 tablespoon water; pinch of black pepper (and/or gochugaru (red chili pepper flakes))